Dynamics of H₂O ligands and ReO₄⁻ anions at the phase transition in [Mn(H₂O)₂](ReO₄)₂ studied by complementary spectroscopic methods

Abstract

The vibrational and reorientational motions of H2O ligands and ReO4− anions in high‐ and low‐temperature phases were investigated by means of Fourier transform far‐ and mid‐infrared and Raman light‐scattering spectroscopy and neutron scattering (inelastic/quasi‐elastic incoherent neutron scattering) methods. The dynamics of H2O and ReO4− molecules in high‐ (I) and low‐temperature (II) phases was investigated by means of band shape analysis performed for Raman and IR bands. The temperature dependencies of full width at half maximum values of the Raman bands at 874 (νas(ReO)) and 943 cm−1 (νs(ReO)) exhibit changes in their behaviour in the vicinity of transformation, suggesting that the observed phase transition is connected with a change in the reorientational dynamics of ReO4−. However, anions also perform fast (τR ≈ 10−12 to 10−13 s) stochastic reorientational motions in Phase II. The estimated mean value of activation energy for ReO4− anions in the low‐temperature phase is Ea(II) = 8.1 kJmol−1. In infrared band shape analysis it was found that H2O ligands perform fast (picosecond correlation time scale) motions in both phases with a mean value of activation energies of 7.68 kJmol−1. These reorientational motions of H2O ligands do not contribute to the phase transition mechanism. Moreover, the observed phase transition was accompanied by the splitting of some bands, suggesting a reduction in crystal structure symmetry. Quasi‐elastic neutron scattering measurements furnished evidence that H2O motion in Phase I can be fairly accurately described by a simple model of 180° jumps around a twofold axis within a picosecond time scale. Additionally, infrared, Raman light‐scattering spectra were calculated using the density functional theory method for the isolated equilibrium model Mn(H2O)2(ReO4)2, and qualitative agreement with the experimental data was achieved. The theoretical vibrational spectra of the title compound were interpreted by means of potential energy distributions using the VEDA 4 program.

Thermal, structural, and spectroscopic characterization of an ionic [Cu(H2O)4](ReO4)2 compound in a wide temperature range

“…The phase transitions (PT) observed in these compounds are of a very interesting nature and also have a different mechanism. For example, for [Mn(H 2 O) 2 ](ReO 4 ) 2 , the phase transition in T C is evidently related to change of the crystal structure and distortance reorientational motions of ReO 4 − [4, 8]. The quasi‐elastic scattering spectra (QENS) and infrared spectra (IR) clearly confirm that the observed PT for [Ba(H 2 O) 3 ](ReO 4 ) 2 ⋅H 2 O complex is connected with slowing down of the reorientation dynamics of water molecules [9].…”

“…Furthermore, they can be used as thermal energy storage/conversion materials (PCM). Looking from the scientific perspective in this type of compounds, various molecular motions of ligands and anions are possible, and finally, they form interesting hydrogen bonds patterns [1–5]. The discussed investigations belong to the fundamental research.…”

“…Some ionic compounds containing H 2 O ligands and ReO 4 − anions in their structure possess one low–temperature phase transition in a solid state [4, 5, 8–10]. The phase transitions (PT) observed in these compounds are of a very interesting nature and also have a different mechanism.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part XIII

“…de Toledo and co‐workers reported high pressure and temperature effects on the molecular crystal 2‐amino‐5‐methyl‐1,3,4‐thiadiazole using Raman and IR spectroscopy combined with DFT at the condensed phase and for the isolated molecule in gas phase . J. Hetmanczyk and L. Hetmanczyk studied the dynamics of H 2 O ligands and ReO 4 − anions at the phase transition in [Mn(H 2 O) 2 ](ReO 4 ) 2 by complementary spectroscopic methods . Jiang et al carried out Raman spectroscopic measurements of ν 1 band of hydrogen sulfide over a wide range of temperature and density in fused silica optical cells .…”

“…[185] (ReO 4 ) 2 by complementary spectroscopic methods. [186] Jiang et al carried out Raman spectroscopic measurements of ν 1 band of hydrogen sulfide over a wide range of temperature and density in fused silica optical cells. [187] Kirtley and co-workers compared pulsed and continuous lasers for high-temperature Raman measurements of anhydrite.…”